1. Field of the Invention
The present invention relates to glass-ceramics having a desired hardness and being excellent in mechanical strength and machinability. Further, the present invention relates to a process for producing glass-ceramics having a desired hardness and being excellent in mechanical strength, machinability and light transmission property.
Further, the present invention relates to glass-ceramics having a desired hardness and being excellent in mechanical strength, machinability and chemical durability. Further, the present invention relates to a process for producing glass-ceramics having a desired hardness and being excellent in mechanical strength, machinability, chemical durability and light transmission property.
These kinds of glass-ceramics are useful as materials for an artificial dental crown and mold materials (mold materials for producing plastic articles, mold materials for producing ophthalmic lenses, and the like).
2. Description of the Prior Art
In recent years, machinable ceramics produced by precipitating a mica crystal have been used as a ceramic material, such as a material for a dental crown, more beautiful and more excellent in biocompatibility than a metal material.
For example, ceramics produced by precipitating mica tetrasilicofluoride (KMg.sub.2.5 Si.sub.4 O.sub.10 F.sub.2) from glass mainly containing SiO.sub.2, MgO, MgF.sub.2, K.sub.2 O, ZrO.sub.2 and Al.sub.2 O.sub.3 have been described in Japanese Patent Unexamined Publication No. Sho-59-50046 (hereinafter referred to as "Prior Art 1").
The ceramics are provided to improve stain resistance through addition of a small quantity of Al.sub.2 O.sub.3 or ZrO.sub.2.
Further, ceramics improved in mechanical strength, produced by precipitating mica (Na.Mg.sub.3.(Si.sub.3 AlO.sub.10)F.sub.2), .beta.-eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) and .beta.-spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) from glass mainly containing SiO.sub.2, MgO, Na.sub.2 O, Li.sub.2 O, ZrO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3 and fluorine have been described in Japanese Patent Unexamined Publication No. Sho-62-70244 (hereinafter referred to as "Prior Art 2").
On the other hand, crystallized glass produced by precipitating potassium fluoro-richterite glass containing components being present within the ranges: 50 to 70% by weight of SiO.sub.2, 4 to 15% by weight of CaO, 8 to 25% by weight of MgO, 2 to 9% by weight of Na.sub.2 O and 2 to 12% by weight of K.sub.2 O has been disclosed in Japanese Patent Unexamined Publication No. Sho-59-207850 (hereinafter referred to as "Prior Art 3"). According to the Prior Art 3, a metastable-phase mica crystal is formed in a temperature range from 600.degree. to 800.degree. C. before precipitation of potassium fluoro-richterite, and then potassium fluoro-richterite is formed by heating the metastable-phase mica crystal to a temperature not lower than 800.degree. C.
However, the bending strength of the machinable ceramics described in the Prior Art 1 (Japanese Patent Unexamined Publication No. Sho-59-50046) is about 1500 kg/cm.sup.2. There arises a problem in that cutting a tooth portion by 1 mm or more is required for securing strength in the case where the machinable ceramics are used as an artificial dental crown. Accordingly, the portions where the ceramics can be used are restricted within narrow scopes. Further, because the ceramics are not sufficient in bending strength, sufficient care must be taken at the time of producing or mounting of an artificial dental crown, compared with the case of a metal material.
The bending strength of the machinable ceramics in the Prior Art 2 (Japanese Patent Unexamined Publication No. Sho-62-70244) is about 2200 kg/cm.sup.2, which is higher than that of the machinable ceramics in the Prior Art 1. However, the ceramics have a disadvantage in that the hardness of the ceramics is more than that of a natural tooth. Accordingly, in the case where the machinable ceramics are used as an artificial dental crown, a problem arises in that the natural tooth is rather worn out because the artificial crown is harder than the natural tooth.
The glass-ceramics described in the Prior Art 3 (Japanese Patent Unexamined Publication No. Sho-52-207850) have a disadvantage in that it is difficult to precipitate stably a mica crystal from the glass composition defined in the Prior Art 3 because the mica crystal is precipitated as a metastable phase. This is because glass containing a large amount (50 to 70% by weight) of SiO.sub.2 in the Prior Art 3 is stabilized at the time of crystallization to thereby make it difficult to precipitate a large amount of fine mica crystal.
Further, in the case where the glass-ceramics are used as a dental crown, the crown is left for a long time in the oral cavity being present under a severe condition. Accordingly, materials excellent in both mechanical strength and chemical durability have been desired.
Further, from the point of view of beauty or the like, materials having light transmission property have been desired.